Seqlock
是在阅读 P1478R1: Byte-wise atomic memcpy 发现的它。
wowotech Linux内核同步机制之（六）：Seqlock
普通的spin lock对待reader和writer是一视同仁，RW spin lock给reader赋予了更高的优先级，那么有没有让writer优先的锁的机制呢？答案就是seqlock。
wikipedia Seqlock
A seqlock (short for sequence lock) is a special locking mechanism used in Linux for supporting fast writes of shared variables between two parallel operating system routines. The semantics stabilized as of version 2.5.59, and they are present in the 2.6.x stable kernel series.
stackoverflow Why rwlock is more popular than seqlock in linux kernel? [closed]
A seqlock has a strong limitation, that readers should correctly work with inconsistent data.
Not every processing algorithm allows inconstistent data. In most cases, such data can only be numbers: integers, booleans, etc. They rarely can be pointers, because a stale pointer may point to the memory which is already freed, so dereferencing such pointer is no-no.
Locks (and rw-locks among them) doesn't have "inconsitent data" limitations, so they can be used in much more cases.
Example of inconstisten data under seqlock
Assume there are two structure's fields protected by the single seqlock. The first field, a is incremented by each "write", the second field, b is decremented by each "write". Both fields initially are 0.
On may assume, that a reader will always find a + b to be 0.
But in case of seqlock, this is not true. E.g., between reading a and b it could a "write", so a value will be old, and b value will be new, and a + b gives -1.